The invention relates to a drive arrangement for precision positioning of two relatively movable members such as a work table on a support plate, that is, for angular or rectilinear position adjustment of the work table by means of a harmonic-type drive arrangement coupling the support plate and the work table with one another.
Such angular positioning arrangements are known as indexing tables. They have coupling drive arrangements being adapted to permit the generation of circle or angle divisions to predetermined adjustable values. Repeatability and accurate adjustability of the graduation of the angular settings are highly important requirements for such arrangements. It is also necessary that adjustment of small graduations on the order of angles of less than 1.degree. can be accurately performed and such adjustments should be accurately maintained even against large forces. It is further necessary that relatively heavy workpieces can be supported and rotated on such positioning arrangements which requires relatively high drive torques.
On such indexing tables usually both the table plate and the support plate are provided with radial gear structures. For angular position adjustment the table plate is usually raised out of engagement with the support plate and rotated to the desired angular position and then again lowered into firm engagement with the support plate. The angular spacing of such tables is determined by the number of teeth of the gear structures which number however cannot be made to be too large since appropriate seating engagement of the indexing table with sufficient engagement forces is required. The raising of the table is avoided in a design known from U.S. Pat. No. 3,714,848 wherein the support plate and indexing table are provided with radial gears with teeth which are inclined toward one another at an acute angle and adapted to receive balls for locking the indexing table with the support plate. The balls however do not only lock the indexing table with the support plate but also insure their accurate positioning relative to one another without any play.
The state of the art also includes an arrangement with a harmonic drive including a cylinder provided with an internal gear structure and a resilient sleeve which has an outer gear structure and receives an elliptic rotor which forces the outer gear structure on the resilient sleeve at opposite sides into engagement with the internal gear structure of the cylinder. The resilient sleeve is stationary and its gear structure has fewer teeth (at least two) than the cylinder's gear structure for large drive speed reduction.
Such an arrangement provides for fine adjustability; however since the sleeve which is part of the drive structure is resilient and engages the cylinder only in a small area, holding torque is limited and accuracy suffers if the load to be retained is relatively large.